Many of the lens barrels which are provided in imaging devices such as film cameras, digital cameras, and camcorders have an internal focusing lens for focusing a subject image onto a film or an imaging element. Some lens barrels have an internal zoom lens for changing the size of the subject image.
In order to focus a subject image and change the subject size, a focusing lens or a zoom lens is retained so as to be movable along an optical axis direction within the lens barrel.
FIGS. 11(a) and (b) show a holder 60 and a lens frame 61 which are provided in a conventional lens barrel. FIG. 11(a) is a perspective view, whereas FIG. 11(b) is a cross-sectional view showing an I-I′ cross section in FIG. 11(a). A lens is retained in the center of the lens frame 61. For example, Patent Document 1 discloses the same construction as the illustrated construction.
The lens frame 61 has a hole for allowing a slide guide 62 to extend therethrough. As the slide guide 62 is movably fitted into the hole, the lens frame 61 moves only along the optical axis direction (J-J′ direction; FIG. 11(b)) of the lens. In other words, the direction of move of the lens frame 61 is restricted by the slide guide 62 so as to be along the J-J′ direction.
Between the lens frame 61 and the holder 60, a spring 63 is provided such that the slide guide 62 is placed through the spring 63. The spring 63 applies an energizing force to the lens frame 61 along an E-E direction shown in FIG. 11(a), which direction is parallel to the J-J′ direction.
The spring 63 retained on the slide guide 62 applies an energizing force to the lens frame 61 in the direction of J, and a driving force from a lens driving mechanism 64, e.g., a motor, is applied thereto. The lens frame 61 is driven along the J-J′ direction by the driving force from the lens driving mechanism 64.
FIG. 11(c) shows a relationship between a guide span K, a length L of the spring 63, and a height N from the plane of the holder 60 to an upper face of a guide section 70. The guide section 70 corresponds to a portion at which the lens frame 61 is movably fitted around the slide guide 62. The guide span K is a length of the guide section 70 along the J-J′ direction. For example, the guide span K is about 5 mm, and the length L of the spring 63 when compressed is about 2 mm. As a result, the height N is about 7 mm.
A length M from the plane of the holder 60 to the lower face of the guide section 70 is the same as the length L of the spring 63. Therefore, the height N from the plane of the holder 60 to the upper face of the guide section 70 is equal to a sum of the length L of the spring 63 and the guide span K. The length L of the spring 63 is determined depending on the length M from the plane of the holder 60 to the lower face of the guide section 70.
Generally speaking, both of the guide span K and the length L of the spring 63 should preferably be long. As the guide span K becomes longer, the precision with which the guide section 70 is guided by the slide guide 62 increases. As the length L of the spring 63 becomes longer, the load when compressing the spring can be made smaller.
On the other hand, the height N from the plane of the holder 60 to the upper face of the guide section 70 should preferably be small. The reason is that reducing the height of the lens barrel will realize thinness.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2000-81557